In the world of microarrays, biological molecules (e.g., oligonucleotides, polypeptides and the like) are placed onto surfaces at defined locations for potential binding with target samples of nucleotides or receptors. Microarrays are miniaturized arrays of biomolecules available or being developed on a variety of platforms. Much of the initial focus for these microarrays have been in genomics with an emphasis of single nucleotide polymorphisms (SNPs) and genomic DNA detection/validation, functional genomics and proteomics (Wilgenbus and Lichter, J. Mol. Med. 77:761, 1999; Ashfari et al., Cancer Res. 59:4759, 1999; Kurian et al., J. Pathol. 187:267, 1999; Hacia, Nature Genetics 21 suppl.:42, 1999; Hacia et al., Mol. Psychiatry 3:483, 1998; and Johnson, Curr. Biol. 26:R171, 1998).
There are, in general, three categories of microarrays (also called “biochips” and “DNA Arrays” and “Gene Chips” but this descriptive name has been attempted to be a trademark) having oligonucleotide content. Most often, the oligonucleotide microarrays have a solid surface, usually silicon-based and most often a glass microscopic slide. Oligonucleotide microarrays are often made by different techniques, including (1) “spotting” by depositing single nucleotides for in situ synthesis or completed oligonucleotides by physical means (ink jet printing and the like), (2) photolithographic techniques for in situ oligonucleotide synthesis (see, for example, Fodor U.S. Pat. No. '934 and the additional patents that claim priority from this priority document, (3) electrochemical in situ synthesis based upon pH based removal of blocking chemical functional groups (see, for example, Montgomery U.S. Pat. No. 6,092,302 the disclosure of which is incorporated by reference herein and Southern U.S. Pat. No. 5,667,667), and (4) electric field attraction/repulsion of fully-formed oligonucleotides (see, for example, Hollis et al., U.S. Pat. No. 5,653,939 and its duplicate Heller U.S. Pat. No. 5,929,208). Only the first three basic techniques can form oligonucleotides in situ e.g., building each oligonucleotide, nucleotide-by-nucleotide, on the microarray surface without placing or attracting fully formed oligonucleotides.
With regard to placing fully formed oligonucleotides at specific locations, various micro-spotting techniques using computer-controlled plotters or even ink-jet printers have been developed to spot oligonucleotides at defined locations. One technique loads glass fibers having multiple capillaries drilled through them with different oligonucleotides loaded into each capillary tube. Microarray chips, often simply glass microscope slides, are then stamped out much like a rubber stamp on each sheet of paper of glass slide. It is also possible to use “spotting” techniques to build oligonucleotides in situ. Essentially, this involves “spotting” relevant single nucleotides at the exact location or region on a slide (preferably a glass slide) where a particular sequence of oligonucleotide is to be built. Therefore, irrespective of whether or not fully formed oligonucleotides or single nucleotides are added for in situ synthesis, spotting techniques involve the precise placement of materials at specific sites or regions using automated techniques.
Another technique involves a photolithography process involving photomasks to build oligonucleotides in situ, base-by-base, by providing a series of precise photomasks coordinated with single nucleotide bases having light-cleavable blocking groups. This technique is described in Fodor et al., U.S. Pat. No. 5,445,934 and its various progeny patents. Essentially, this technique provides for “solid-phase chemistry, photolabile protecting groups, and photolithography . . . to achieve light-directed spatially-addressable parallel chemical synthesis.”
The electrochemistry platform (Montgomery U.S. Pat. No. 6,092,302, the disclosure of which is incorporated by reference herein) provides a microarray based upon a semiconductor chip platform having a plurality of microelectrodes. This chip design uses Complimentary Metal Oxide Semiconductor (CMOS) technology to create high-density arrays of microelectrodes with parallel addressing for selecting and controlling individual microelectrodes within the array. The electrodes turned on with current flow generate electrochemical reagents (particularly acidic protons) to alter the pH in a small “virtual flask” region or volume adjacent to the electrode. The microarray is coated with a porous matrix for a reaction layer material. Thickness and porosity of the material is carefully controlled and biomolecules are synthesized within volumes of the porous matrix whose pH has been altered through controlled diffusion of protons generated electrochemically and whose diffusion is limited by diffusion coefficients and the buffering capacities of solutions. However, in order to function properly, the microarray biochips using electrochemistry means for in situ synthesis has to alternate anodes and cathodes in the array in order to generated needed protons (acids) at the anodes so that the protons and other acidic electrochemically generated acidic reagents will cause an acid pH shift and remove a blocking group from a growing oligomer.
Gene Assembly
The preparation of arbitrary polynucleotide sequences is useful in a “post-genomic” era because it provides any desirable gene oligonucleotide or its fragment, or even whole genome material of plasmids, phages and viruses. Such polynucleotides are long, such as in excess of 1000 bases in length. In vitro synthesis of oligonucleotides (given even the best yield conditions of phosphoramidite chemistry) would not be feasible because each base addition reaction is less than 100% yield. Therefore, researchers desiring to obtain long polynucleotides of gene length or longer had to turn to nature or gene isolation techniques to obtain polynucleotides of such length. For the purposes of this patent application, the term “polynucleotide” shall be used to refer to nucleic acids (either single stranded or double stranded) that are sufficiently long so as to be practically not feasible to make in vitro through single base addition. In view of the exponential drop-off in yields from nucleic acid synthesis chemistries, such as phosphoramidite chemistry, such polynucleotides generally have greater than 100 bases and often greater than 200 bases in length. It should be noted that many commercially useful gene cDNA's often have lengths in excess of 1000 bases.
Moreover, the term “oligonucleotides” or shorter term “oligos” shall be used to refer to shorter length single stranded or double stranded nucleic acids capable of in vitro synthesis and generally shorter than 150 bases in length. While it is theoretically possible to synthesize polynucleotides through single base addition, the yield losses make it a practical impossibility beyond 150 bases and certainly longer than 250 bases.
However, knowledge of the precise structure of the genetic material is often not sufficient to obtain this material from natural sources. Mature cDNA, which is a copy of an mRNA molecule, can be obtained if the starting material contains the desired mRNA. However, it is not always known if the particular mRNA is present in a sample or the amount of the mRNA might be too low to obtain the corresponding cDNA without significant difficulties. Also, different levels of homology or splice variants may interfere with obtaining one particular species of mRNA. On the other hand many genomic materials might be not appropriate to prepare mature gene (cDNA) due to exon-intron structure of genes in many different genomes.
In addition, there is a need in the art for polynucleotides not existing in nature to improve genomic research performance. In general, the ability to obtain a polynucleotide of any desired sequence just knowing the primary structure, for a reasonable price, in a short period of time, will significantly move forward several fields of biomedical research and clinical practice.
Assembly of long arbitrary polynucleotides from oligonucleotides synthesized by organic synthesis and individually purified has other problems. The assembly can be performed using PCR or ligation methods. The synthesis and purification of many different oligonucleotides by conventional methods (even using multi-channel synthesizers) are laborious and expensive procedures. The current price of assembled polynucleotide on the market is about $12-25 per base pair, which can be considerable for assembling larger polynucleotides. Very often the amount of conventionally synthesized oligonucleotides would be excessive. This also contributes to the cost of the final product.
Therefore, there is a need in the art to provide cost-effective polynucleotides by procedures that are not as cumbersome and labor-intensive as present methods to be able to provide polynucleotides at costs below $1 per base or 1-20 times less than current methods. The present invention was made to address this need.